Conductive resin molded product having insulating skin and method for forming the same

ABSTRACT

The present invention relates to a conductive resin molded product having an insulating skin produced by a composite comprising a non-conductive resin and conductive material. The invention also provides a method for producing the conductive resin molded product.

This application claims priority to a Japanese patent application No.2002-347673 filed Nov. 29, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a conductive resin molded producthaving an insulating skin produced by a composite consisting ofnonconductive resin and conductive material, and relates to a method forproducing the same.

2. Detailed Description of the Prior Art

According to a conventional method, nonconductive resin is blended withconductive material such as carbon black and carbon fiber, or metalpowder and metal fiber and the composite is then molded to produce aconductive resin molded product. (For example, refer to Non-patentliterature 1 below).

Moreover, some conductive molded products are produced byinjection-filling a mold with conductive resin in which a conductivecompounding material such as metal fiber or metal powder is blended withnonconductive resin. (For example, refer to Patent literature 1 below).

Non-patent literature 1: Ebihara, “Handbook of New Polymer Materials” P.69 to 74, Maruzen Co., Ltd. September 20, 1989.

Patent literature 1: The Japanese Patent Laid-Open No. 1993-131445, P.5.

Conventionally, a conductive resin molded product has been produced byblending a conductive compounding material with a nonconductive resin toprovide a resin with conductivity. However, as described in Non-patentliterature 1 and Patent literature 1, most of the conventionalconductive resins have adopted carbon black, carbon fiber, metal powder,or metal fiber which has remarkably large particles compared with themolecules of the resin as the conductive compounding material. When suchconductive compounding material is blended to the extent that the resinis able to have conductivity, the molded product has conductivity evenon its surface; therefore, insulation treatment of the surface isnecessary depending on its use.

Moreover, the shape of the molded product tends to be restricted sincethe properties of the resin such as lightweight, flexibility,moldability, and processability are deteriorated, causing a hindrance inthe production of the molded product by injection molding and loweringmechanical strength and the like. As a result, there is a problem inadopting the above-mentioned technique to a product with a complexshape, even when used as a magnetic wave shield material.

SUMMARY OF THE INVENTION

The present invention is devised in order to solve the problems of theconventional conductive resin molded product as mentioned above. Thepurpose of the present invention is to expand the use of the conductiveresin molded product and to provide a new conductive resin moldedproduct having a new insulating skin which is usable as a base materialfor parts of electronic equipment such as laminated connectors as wellas a method for producing the conductive resin molded product. For thispurpose, a carbon nano material is adopted as a conductive compoundingmaterial to make the surface of the conductive resin molded productnonconductive.

The conductive resin molded product for the above-mentioned purposeaccording to the present invention comprises a resin insulating skin anda conductive core covered with said skin and is composed of a compositecontaining a non-conductive resin and a carbon nano material. The resininsulating skin is obtainable from molding said composite by controllingan amount of the carbon nano material to be composited with thenon-conductive resin.

Moreover, a molding method for producing a conductive resin moldedproduct according to the present invention comprises the steps of;

-   plasticizing a composite material containing a non-conductive resin    and a carbon nano material; and-   injection molding thus plasticized material into a mold cavity to    produce the conductive resin molded product comprising a resin    insulating skin and a conductive core covered with said skin. In the    method, an amount of the carbon nano material to be composited with    the non-conductive resin is controlled so as to form the resin    insulating skin in contact with a cavity face during said injection    molding.

A ratio of the carbon nano material to be composited with saidnon-conductive resin does not exceed 15 weight % based on the compositeto form the resin insulating skin in contact with a cavity face duringsaid injection molding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross sectional view of a part of a conductiveresin molded product having an insulating skin according to the presentinvention.

FIG. 2 is an explanatory illustration of behavior of a compositeconductive material flowing in the cavity up to completion of thefilling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an enlarged cross sectional view of a part of a conductiveresin plate mentioned as an example of the present invention. Thereference numeral 1 is an insulating resin skin, and the referencenumeral 2 a conductive core coated with the insulating skin 1. Theconductive resin plate is obtained by injection molding a compositeconductive material with which a carbon nano material is blended. It isa flat plate with a thickness of 1.5 to 3.0 mm and an upper face area of30 to 40 cm², and consists of the insulating skin 1 with a thickness of0.1 to 0.2 mm and the core 2 having conductivity brought by the carbonnano material inside of it. The surface of the conductive resin platehas an electrical resistance of 10¹⁰ Ωcm or more.

Even if the above-mentioned conductive resin plate is insulated by thesurface resin, an end of a conductive part breaks through the insulatingskin 1 and reaches the core 2 when the part sticks into the resin plate.Therefore, the part becomes electrically connected with the conductivecore 2. Such a conductive resin plate can be used as an electromagneticwave shield material since it has an insulating skin, and can also beused as a base material for a laminated connector. The conductive resinplate is also applicable to many other uses.

Since conductive core 2 is coated with insulating skin 1 in use as anelectromagnetic wave shield material, it is unnecessary to take intoaccount electric damage caused by contact with other electronicequipment, parts, or the like. Moreover, since the surface is made byresin, any surface treatment such as mirror finishing of the surface andembellishment is also easily carried out by a conventional treatmentmethod employed for the resin previously.

Although an illustration is omitted here, the laminated connector caneasily be manufactured by adhering a necessary number of stack sheetstogether into a laminated plate, cutting this at equal intervals intoplate bodies in which the insulating skin and the conductive core arealternately placed, and only cutting the plate body to a necessarydimension in the direction orthogonal to the insulating skin. Thus, aconnector constituted of the conductive cores of the number equal to thestacked sheets is formed into a laminated type connector divided by theinsulating skins.

In a conventional laminated connector made of rubber, after a thin filmof rubber provided with conductivity is alternately laminated with athin film of insulating rubber and the two films are fixed together, thelaminate is then cut to manufacture a rubber laminated connector. On theother hand, using the conductive resin molded product having insulatingskin 1 eliminates the alternate lamination of the insulating skins, andthe laminated plate is easily formed by mutual adhesion between theinsulating skins. Therefore, manufacture is simpler than that usingrubber, and a laminated type resin connector, which has been regarded asdifficult to manufacture, can be provided at a lower cost.

Moreover, the carbon nano material is an ultrafine particulate andpresent in a blending quantity not exceeding 15 weight %. Since it doesnot damage the characteristics of the resin, injection molding can beperformed under conditions set according to the resin. Specialtechniques are not required for molding and there is little change inproperties. Therefore, the resin does not lose its characteristics inthe molding process, and a conductive resin plate having furtherimproved dimensional accuracy can be obtained as the base material forparts.

In order to produce the above-mentioned conductive resin plate byinjection molding, a composite conductive material blending a carbonnano material, not exceeding 15 weight %, with a nonconductive resin isused. As the nonconductive resin, a thermoplastic resin used as amolding material, for example, polyethylene, polyester, polyamide,polycarbonate, ABS resin, and liquid crystal polymer can be used.

Moreover, as a carbon nano material to be blended with the nonconductiveresin, nano fibers (having a diameter of 50-200 nm, preferably, 80-150nm, and an aspect ratio of 100-1000), nano carbon tubes (having adiameter of 1-50 nm, preferably, 10-50 nm, and an aspect ratio of100-1000), fullerene (having a diameter of 0.7-1 nm), or the like can beused. Since they are more ultrafine particulate than the metal powderand metal fiber which have been blended as the conductive material inprevious composite conductive materials, they have good comformabilityto the resins, and have a good dispersion efficiency by kneading. As aresult, the properties of the resins such as flexibility, moldability,and processability are not lost.

In the aforementioned case, it is most preferable that such compositeconductive material is pelletized beforehand and supplied to aninjection molding machine. However, there is no difficulty in themolding even if both of the resin and carbon nano material aresufficiently kneaded by a kneader and then supplied to the injectionmolding machine. Therefore, the composite conductive material may besupplied by either method.

The molding conditions of the injection molding machine such astemperature of a heating cylinder, cooling temperature of a productmold, screw speed, injection speed and pressure are arbitrarily setaccording to the kind of resin adopted there. After the compositeconductive material supplied from a hopper into the heating cylinderwith a built-in screw is plasticized (melted and kneaded) by ordinaryinjection molding operation, the material is measured and then filled byinjection into the mold by forward stroke of the screw.

Each illustration in FIG. 2 shows the behavior of the molten body 13 ofthe composite conductive material flowing in the cavity 12 of the mold11 before completing the filling, and as shown in the illustration (A),the molten body 13 flows at the highest speed in the center part, andflows slower as it approaches a cavity surface 12 a. Moreover, as shownin the illustration (B), the molten body is increased in viscosity dueto cooling of the mold 11, becomes resistant to flow, and the resin iscooled and solidified into the surface layer (the skin).

From this difference in flow, a velocity gradient, namely, a rate ofshear arises between the center part of the molten body 13 and thecontact part with the cavity surface 12 a. Thus, the resin at the cavitysurface 12 a, which becomes cooled and solidified, is extended in thedirection of the flow because of the large shearing stress applied on itfrom the molten body 13 being press-fitted. At the same time, the carbonnano material on the skin side is also pulled and aligned in thedirection of the flow, and also becomes easily centralized in the centerof the molten body away from the skin 13 a.

On the other hand, since the core 13 b is little influenced by theshearing stress and the carbon nano material exhibits anisotropy,conductivity appears. It is difficult to obtain conductivity by usingfullerene, but an effect is obtained by using a carbon nano materialtogether with fullerene. This phenomenon is conditional on the blendingquantity of carbon nano material; the blending quantity is preferred tobe 5-15 weight %. In the case of a blending quantity exceeding 15 weight%, conductivity appears also in the skin 13 a, and this makes itdifficult to form an insulating skin 13 a out of the resin. After havingcompleted filling the resin, the resin is cooled and solidified into theconductive resin plate comprising the skin 13 a of the resin havingnon-conductivity and the conductive core 13 b coated with the skin 13 aas shown in the illustration. Namely, the insulating skin formed out ofresin and the core coated with the insulating skin are formed by thedifference in fluidity between the resin and the carbon nano materialflowing in the cavity and shearing stress on the cavity surface obtainedby controlling a blending quantity of a carbon nano material.

EXAMPLE OF THE EMBODIMENT

Conductive resin molded product Form and dimensions; Flat plate(rectangular shape), Plate thickness: 2.0 mm Plane area of its upperface: 36 cm² Resin; Polypropylene Compounding ingredient; Carbon nanotube, 10 nm diameter, 1 to 10 μm long Blending quantity; 10 weight %Conductivity (volume Surface: 10¹⁰ Ωcm or more, resistivity); Inside:10³ Ωcm or less Injection molding machine; PS40 (manufactured by NISSEIPLASTIC INDUSTRIAL CO., LTD.) Molding conditions; Plasticizingtemperature 210° C. Injection speed 100 mm/s Injection pressure 100 MPaMold temperature 30° C.

1. A conductive resin molded product comprising: a resin insulating skinobtained from molding a composite containing a non-conductive resin anda carbon nano material by controlling an amount of the carbon nanomaterial to be composited with the non-conductive resin; and aconductive core covered by said resin insulating skin, wherein a ratioof a carbon nano material to be composited with said non-conductiveresin is less than about 15 weight percent based on the composite.
 2. Amolding method for producing a conductive resin molded product, saidmethod comprising the steps of: plasticizing a composite materialcontaining a nonconductive resin and a carbon nano material; andinjection molding thus plasticized material into a mold cavity toproduce the conductive resin molded product comprising a resininsulating skin and a conductive core covered with said skin, an amountof the carbon nano material to be composited with the non-conductiveresin being controlled so as to form the resin insulating skin incontact with a cavity face during said injection molding, wherein aratio of a carbon nano material to be composited with saidnon-conductive resin is less than about 15 weight percent based on thecomposite.